How quantum are classical spin ices?

TL;DR

This paper investigates the quantum effects in classical spin ice materials Dy2Ti2O7 and Ho2Ti2O7, concluding that quantum effects are negligible at relevant temperatures due to weak high-rank multipolar interactions.

Contribution

It provides a microscopic analysis showing that quantum effects in these spin ices are minimal, challenging the assumption that they are purely classical and highlighting the weak role of super-exchange.

Findings

Quantum effects are below experimentally relevant temperatures.

High-rank multipolar interactions are generated very weakly.

Classical Ising model remains valid for these materials at low temperatures.

Abstract

The pyrochlore spin ice compounds Dy2Ti2O7 and Ho2Ti2O7 are well described by classical Ising models down to very low temperatures. Given the empirical success of this description, the question of the importance of quantum effects in these materials has been mostly ignored. We show that the common wisdom that the strictly Ising moments of Dy3+ and Ho3+ imply strictly Ising interactions is too naive; a more complex argument is needed to explain the close agreement between theory and experiment. From a microscopic picture of the interactions in rare-earth oxides, we show that the high-rank multipolar interactions needed to induce quantum effects are generated only very weakly by super-exchange. Using this framework, we formulate an estimate of the scale of quantum effects in Ho2Ti2O7 and Dy2Ti2O7, finding it to be well below experimentally relevant temperatures. We discuss the implications of these results for realizing quantum spin ice in other materials.